Added work on making namespaces threadsafe.

src/c/payloads/hashtable.h

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+/**
+ *  payloads/hashtable.h
+ *
+ *  an ordinary Lisp hashtable - one whose contents are immutable.
+ *
+ *  Can sensibly sit in any pso from size class 6 upwards.
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#ifndef __psse_payloads_hashtable_h
+#define __psse_payloads_hashtable_h
+
+#include "memory/pointer.h"
+
+/**
+ * @brief Tag for an ordinary Lisp hashtable - one whose contents are immutable.
+ * \see NAMESPACETAG for mutable hashtables.
+ */
+#define HASHTABLETAG "HTB"
+/**
+ * The payload of a hashtable. The number of buckets is assigned at run-time,
+ * and is stored in n_buckets. Each bucket is something ASSOC can consume:
+ * i.e. either an assoc list or a further hashtable.
+ */
+struct hashtable_payload {
+    struct cons_pointer hash_fn;    /* function for hashing values in this hashtable, or `NIL` to use
+                                     the default hashing function */
+    struct cons_pointer write_acl;  /* it seems to me that it is likely that the
+                                     * principal difference between a hashtable and a
+                                     * namespace is that a hashtable has a write ACL
+                                     * of `NIL`, meaning not writeable by anyone */
+    uint32_t n_buckets;             /* number of hash buckets */
+    uint32_t unused;                /* for word alignment and possible later expansion */
+    struct cons_pointer buckets[];  /* actual hash buckets, which should be `NIL`
+                                     * or assoc lists or (possibly) further hashtables. */
+};
+
+#endif